Tubulysins A and D are members of a group of antimitotic peptides that were originally isolated form the myxobacterial strains Archangium gephyra and Angiococcus disciformis. Both compounds inhibit the growth of cancer cell lines at very low concentrations, whereby tubulysin D is approximately 10 folds more potent than tubulysin A. Treatment of mammalian cells with tubulysins induces a cell cycle arrest at the G2M phase that followed by induction of apoptosis (Khalil, M. et al., Chembiochem. 7, 678-683 (2006); Sasse, F. et al., Nat. Chem. Biol. 3, 87-89 (2007)). In vitro both drugs inhibit the polymerization of purified microtubule protein and induce the de-polymerization of preassembled microtubules. Previous report demonstrated that tubulysin A strongly interfered with the binding of vinblastine to tubulin in a non-competitive manner (Khalil, M. et al., Chembiochem. 7, 678-683 (2006)).
Certain tubulysin derivatives designated “pretubulysins” were known in the art were firstly observed in extracts from myxobacteria (structures see below). Later they could be chemically synthesized by the Kazmaier group at UDS. For example WO 2008/106080 discloses such pretubulysins having an optionally acylated hydroxyl residue in the central thiazol-containing amino acid residue and/or having an unsaturated C-terminal amino acid residue; and WO2004/046170 mentions fully saturated pretubulysins. However, the apoptotic and cytotoxic activity of these pretubulysins is significant inferior to tubulysin A and D. As the natural abundance of tubulysin A and D is moderate and a synthetic process is not available, there is a strong demand for alternative substances having comparable activity as tubulysin A and D.